Rake receiver and a method of providing a frequency error estimate

ABSTRACT

An input signal is mixed with a scrambling code and with an OVSF code, and then accumulated over a period of time equal to one symbol of the pilot channel before being reset. The resultant complex signals are mixed with the conjugate of complex signals provided by a pilot pattern generator, with the result being fed to a delay line ( 22 ). A multiplier multiplies the complex value at a location P(n) in the delay line ( 22 ) by the conjugate of a complex value at another location P(n+k), and provides the resulting complex number (which is a complex vector whose phase is a coherent measurement of the phase rotation occurring between the symbols corresponding to the two locations) at an output ( 27 ). The real part of the complex signal provided at the output ( 27 ) is provided to a first combiner ( 30 ), and the imaginary part is provided to a second combiner ( 31 ). These combiners ( 30, 31 ) also receive corresponding signals from all of the other fingers (not shown) of the rake receiver. Each combiner ( 30, 31 ) coherently sums all of the signals at its inputs and provides the result to its respective accumulator ( 32, 33 ), which provide coherent filtering. The phase rotation of the combined and accumulated signals is then calculated. Once convergence of the frequency of an LO with the frequency of the received signals is achieved, the value of k is increased.

[0001] This invention relates to a method of providing an estimate ofthe difference in frequency between a carrier of a received signal and alocally-generated version of the carrier. This invention relates also toa rake receiver.

[0002] This invention has application in automatic frequency controlsubsystems in rake receivers.

[0003] In accordance with a first aspect of the invention, there isprovided a method of providing an estimate of the difference infrequency between a carrier of a received signal and a locally-generatedsignal, the method comprising: in each of first and second fingers of arake receiver: providing a first complex signal derived from acorrelation of a received signal with the locally-generated signal overa first period of time; providing a second complex signal derived from acorrelation of the received signal with the locally-generated signalover a second period of time, the second period of time being separatedfrom the first period of time; and determining the product of the firstsignal and the conjugate of the second signal to provide a productsignal; combining the product signals from the first and second fingersto provide a combined signal; using the combined signal to estimate thefrequency error; and dynamically adjusting the spacing between the firstand second periods of time on the basis of the estimated frequencyerror.

[0004] In accordance with a second aspect of the invention, there isprovided a rake receiver, comprising: an oscillator, forlocally-generating an oscillatory signal; an input, for receiving aninput signal; and first and second fingers, each of the fingers havingassociated therewith: a first correlator, arranged to correlate theinput signal with the oscillatory signal over first and second periodsof time to provide, respectively, first and second complex signals; anda multiplier arranged to multiply the first complex signal with theconjugate of the second signal to provide a product signal; the rakereceiver further comprising: a sequencer arranged to cause the firstperiod of time to be separated from the second period of time; acombiner arranged to combine the product signals from the first andsecond fingers to produce a combined signal; and an estimator arrangedto estimate a frequency error of the oscillator on the basis of thecombined signal, the sequencer being arranged to adjust dynamically theseparation of the first and second periods of time on the basis of theestimated frequency error.

[0005] The invention allows exploitation of the diversity offered by thepropagation channel between a transmitter and the rake receiver. Theinvention provides some immunity also to impulsive noise caused by phasestep changes during fades.

[0006] An embodiment of the invention will now be described withreference to the accompanying drawings, of which:

[0007]FIGS. 1 and 2 show schematically parts of a rake receiverimplementing the invention.

[0008] A finger 10 of a rake receiver to which the invention is appliedis shown in FIG. 1. Referring to FIG. 1, the finger 10 comprisesgenerally circuitry forming a traffic channel 11 and circuitry forming apilot channel 12. A mixer 13 in the traffic channel circuitry 11 mixesan input signal, received from a schematic delay line 14, with a codeprovided by a scrambling code generator 15 and with a traffic channelspecific code provided by a first OVSF code generator 16. The resultantsignal is fed to a first accumulator 17 and to a first-in-first-out(FIFO) buffer 18, in a conventional manner. In the pilot channelcircuitry 12, the input signal is mixed, in a second mixer 19, with thecode provided by the scrambling code generator 15 and with a pilotchannel specific code, generated by a second OVSF code generator 20.This mixed signal is then accumulated in a second accumulator 21 over aperiod of time equal to one symbol of the pilot channel before beingreset. The reset period of the second accumulator 21 is aligned withreset period of the OVSF code provided by the first OVSF code generator16. The resultant complex signals are mixed in a pilot mixer 28 with theconjugate of complex signals provided by a pilot pattern generator. Theresultant pilot pattern mixed complex signals are fed to a second delayline 22 and, from there, to a complex multiplier 23 via a coherent phasereference device 24. The complex multiplier 23 multiplies the outputsignals from the traffic channel 11 and the pilot channel 12, the resultbeing provided to a coherent combiner (not shown) along with signalsfrom other fingers (not shown) of the rake receiver.

[0009] The code generators 15, 16 and 20 are symbol locked with eachother. The code generator 15 runs at chip rate, the code generator 16runs at traffic channel symbol rate and the code generator 20 runs atpilot channel symbol rate.

[0010] The complex value at a location P(n) in the second delay line 22is provided to a first input of a multiplier 25. The conjugate of acomplex value at another location P(n+k) in the second delay line 22 iscalculated by a conjugate calculation device 26, and the result providedto a second input of the multiplier 25. The multiplier 25 multiplies thetwo complex numbers it receives, and provides the resulting complexnumber at an output 27. Initially, the value of k is set to 2, thelocation P(n) then corresponding to an accumulation result two symbolssubsequent to the location P(n+2). The second delay line 22 operates ina rolling manner such that, when another accumulation result is providedby the second accumulator 21, this is fed through the second delay line,and the multiplier 25 is then fed with signals from subsequent locationsin the second delay line. A new complex number output is, therefore,provided every symbol.

[0011] The complex number output is a complex vector whose phase is acoherent measurement of the phase rotation occurring between the symbolscorresponding to the locations P(n) and P(n+k). The magnitude of thecomplex vector is proportional to the average power of the accumulationresults from those two symbols.

[0012]FIG. 2 shows a further part of the finger 10 of the rake receiver,with reference numerals retained from FIG. 1 for like elements. Thefinger 10 further comprises first and second combiners 30, 31, whichfeed a respective one of second and third accumulators 32, 33, a phasecomputation device 34 and a low-pass filter 35.

[0013] The real part of the complex signal provided at the output 27 ofthe complex multiplier 25 is provided to a first input of the firstcombiner 30. The imaginary part of the same signal is similarly providedto a first input of the second combiner 31. These combiners 30, 31 alsoreceive corresponding signals from all of the other fingers (not shown)of the rake receiver. Each combiner 30, 31 coherently sums all of thesignals at its inputs and provides the result to its respectiveaccumulator 32, 33, which provide coherent filtering. The phase rotationof the combined and accumulated signals is then calculated by the phasecomputation device 34, which operates in a conventional manner, takinginto account the time separation of the locations P(n) and P(n+k). Sincethe magnitudes of the complex signals are related to the strengths ofthe signals from which they are derived, this arrangement gives weightto the contributions that the fingers make according to the power of thesignal that they receive. The angle of phase rotation calculated by thephase calculation device 34 is filtered by the low pass filter 35 priorto being fed back to control the operating frequency of the receiver'slocal oscillator (not shown) to tend towards that of the receivedcarrier.

[0014] Once convergence is reached, the value of k is increased, so thatthe multiplier 25 receives signals which correspond to accumulationresults spaced further apart in time, for example six symbols apart.This allows more accurate phase, and therefore frequency, error signalsto be calculated. This has the advantage of allowing for a finerresolution in frequency error measurements.

[0015] This invention is also used in providing a frequency errorestimate from signals associated with different base stations in a softhandover scenario. The different fingers within the rake receiverdespread incoming signals transmitted from different base stationtransmitters. Since different base stations have different scramblingcodes, different fingers are provided with different OVSF and scramblingcodes. In this case, first and second fingers each include a respectivecode generator arranged to generate a code different to the codegenerated by the other code generator, i.e. one code corresponds to thepilot channel code from one base station and the other code correspondsto the pilot channel code from another base station. The resultingfrequency error estimate is the difference between the local frequencyreference (the local oscillator) and the average frequency of thecarriers of the base stations transmitters involved in soft handover.Signals contributing to the frequency error estimate are weightedaccording to their signal strengths.

[0016] Although the combining of signals from only two fingers isdescribed, it is expected that the invention will be used to combinesignals from all or most of the fingers of a rake receiver, which may besix or eight fingers for a UMTS radiotelephone rake receiver forexample.

[0017] The invention may be implemented in software, in an applicationspecific integrated circuit (ASIC) or in a field programmable gate array(FPGA), for example.

1. A method of providing an estimate of the difference in frequencybetween a carrier of a received signal and a locally-generated signal,the method comprising: in each of first and second fingers of a rakereceiver: providing a first complex signal derived from a correlation ofa received signal with the locally-generated signal over a first periodof time; providing a second complex signal derived from a correlation ofthe received signal with the locally-generated signal over a secondperiod of time, the second period of time being separated from the firstperiod of time; and determining the product of the first signal and theconjugate of the second signal to provide a product signal; combiningthe product signals from the first and second fingers to provide acombined signal; using the combined signal to estimate the frequencyerror; and dynamically adjusting the spacing between the first andsecond periods of time on the basis of the estimated frequency error. 2.A method according to claim 1, further comprising comparing theestimated frequency error to a threshold, and controlling an operatingmode of the rake receiver on the basis of the comparison.
 3. A methodaccording to either preceding claim, further comprising low-passfiltering successive frequency error estimates to provide a filteredsignal, and controlling the operating frequency of an oscillatorproviding the locally-generated signal on the basis of the filteredsignal.
 4. A rake receiver, comprising: an oscillator, forlocally-generating an oscillatory signal; an input, for receiving aninput signal; and first and second fingers, each of the fingers havingassociated therewith: a first correlator, arranged to correlate theinput signal with the oscillatory signal over first and second periodsof time to provide, respectively, first and second complex signals; anda multiplier arranged to multiply the first complex signal with theconjugate of the second signal to provide a product signal; the rakereceiver further comprising: a sequencer arranged to cause the firstperiod of time to be separated from the second period of time; acombiner arranged to combine the product signals from the first andsecond fingers to produce a combined signal; and an estimator arrangedto estimate a frequency error of the oscillator on the basis of thecombined signal, the sequencer being arranged to adjust dynamically theseparation of the first and second periods of time on the basis of theestimated frequency error.
 5. A rake receiver according to claim 4,further comprising a comparator, the comparator being arranged tocompare the estimated frequency error to a threshold; and a controllerarranged to control an operating mode of the rake receiver on the basisof the comparison result.
 6. A rake receiver according to claim 4 orclaim 5, further comprising a low-pass filter arranged to filtersuccessive frequency error estimations, and to provide the resultingfiltered signal to a frequency control input of the oscillator.
 7. Arake receiver according to any of claims 4 to 6, in which the first andsecond fingers each include a respective code generator arranged togenerate a code different to the code generated by the other codegenerator.